hello there.after some advice please.
got a q7 3l tdi 2014 crca
my brakes feel quite bad to me,like they get on almost at half pedal,pads and discs are good, brake fluid replaced.
for example if car is on idle, pump the brakes,pedal gets hard but if you keep pressure on the pedal,it goes down. any ideas please?

 

Going to give you the deep-dive, as it took me a very long time to figure this out on my own Q7, and there's more than one possible issue or root cause.

If you DIY'd the brakes yourself, then it's very important to use VCDS or similar capable scan tool to run the ABS pump (brake bleeding sequence) to get all the air out of it, as well as the brake lines, etc. There's also two bleed nipples per caliper on these Brembo calipers, and a specific sequence to use during bleeding. When using the VCDS brake bleeding found under 'basic settings' button in VCDS, IIRC, then it'll guide you through the whole brake bleeding process step-by-step. You'll only need to top up the reservoir or check/pump up the power bleeder if using one.

On my own Q7, I had very similar symptoms and chased the braking problem around the car for long time, even bleeding brakes multiple times with power bleeder and cycling ABS pump using VCDS program, to no avail. I looked up tests for master cylinder and brake booster failure, but all those possible failure points checked out fine.

It turned out to be the vacuum line connection was split and leaking vacuum where engine vacuum line connects to the brake booster's vacuum line. On my LHD car, it's on the passenger side, and the connection of those two lines is just outside the firewall in the engine bay.

The brake booster vac line passes through the firewall from the plenum side, and then it almost immediately connects with the engine vacuum line. The failure point is the female connector splits open on the vac line coming from brake booster. The fix is to either replace the entire vac line or simply patch the leak or cut out the bad section and splice in a new piece, etc.

I simply repaired mine using JB-Weld pipe repair tape, and if that ever fails, then I'll cut out the bad section and splice in a new connection, etc. My brake pedal went from floppy to 'normal' just as soon as I wrapped it up.

The other possibility, which is somewhat rare, is that the ABS pump will develop an air leak. That's a fairly expensive part, so don't jump to conclusions about that one; plenty of threads I read where Audi dealer claimed that was problem and replaced the ABS pump and it didn't resolve the issue with the low, spongy brake pedal, or eventual loss of power assistance (pedal gets rock hard when no vacuum servo assistance is present). I experienced same symptoms as you detail, and mine was getting really, really bad, and I had literally looked at every possibility and tested everything twice.

I was so super-frustrated that my car was literally at the point of being unsafe to drive. In the end, I actually took the car in to Audi for a seemingly unrelated fault code/CEL, and they found a coolant hose (the 'crack-pipe') over top of engine had failed and was leaking coolant into valley (covered via warranty), they also discovered I had a vacuum leak. The vacuum leak led to the CEL/fault codes, and that traced directly to the brake booster/engine vac lines junction point was leaking vacuum...the tech actually showed me how he could just pull up on engine vac line and it easily separated from the brake booster line's connection side, and I could see the female plastic connector was badly cracked.

I had scoured every thread I could find on Q7 brakes problems and not run across that as an issue, but there it was...the underlying low brake pedal and gradual loss of braking power assist was finally identified, and it made sense, as it was a leak and not a complete loss of vacuum all at once, which would have resulted in a hard, unyielding pedal with zero assist. YES!!!

Then, of course the Audi adviser handed me their estimate to to perform 'recommended repairs' on it by replacing both vacuum lines, etc., and the final cost was an incredibly high $$$ value. I'm like, no thanks, I don't need you to do anything that's not paid for by the TDI warranty, and I carefully drove it back home and promptly repaired the vacuum leak they'd found for me within 30 minutes. Brakes and pedal felt like brand new again even though I've got 40k miles on the rotors/pads set.

Lastly, there is a vacuum pump on this car, and of course it can fail at some point, but 100% of these Q7 TDIs (and the gassers too) will eventually get a vacuum leak where the two vacuum lines connect; it's just an age/design/heat-cycling thing. The plastic on the female-side of the connection is super thin, so it fatigues and splits open. On my car it resembled a flower bloom and had separated into pieces that resembled flower petals.

To access/work on it, I just had to move the intake air pipe from air cleaner to turbo (I rotated it up at turbo side, as it obscures this vacuum line connection point.